Reusing roasted spices is generally not recommended. They lose their potency and flavor profile after the initial roasting and cooking process.
When you roast spices, you're essentially waking up their essential oils and volatile compounds, which are responsible for their aroma and taste. This process enhances their flavor. However, once these spices are used in a dish and cooked again, those oils and compounds degrade further. The second cooking can result in a muted, less vibrant flavor. Furthermore, spices can absorb moisture and flavors from the dish they were initially used in. Reusing them might introduce unwanted flavors or dilute the intended taste of your new dish. While it might seem economical, the impact on the final flavor is usually not worth it. Freshly roasted and ground spices will always provide the best and most consistent results.
To maximize the flavor of your spices and avoid waste, roast only the amount you need for a particular recipe. Store leftover roasted spices in an airtight container in a cool, dark place to preserve their flavor for as long as possible, but remember they will still degrade over time.